I build such a scenario for a colleague. Instead of using MPLS I use a WAN-link (doesn't matter).
I only backup ASA 2, but it should run in both ways.
Routing protocol RIPv2
Internet
|
ASA 1
|
inside - 192.168.16.0/21
|
WAN
|
inside - 192.168.32.0/21
|
ASA 2
|
internet
NAT configuration on ASA 1
--------------------------
nat (inside) 1 0.0.0.0 0.0.0.0
global (outside) 1 interface
route outside 0.0.0.0 0.0.0.0 81.x.x.x
configuration ASA 2
-------------------
global (outside) 1 interface
nat (inside) 1 0.0.0.0 0.0.0.0
route outside 0.0.0.0 0.0.0.0 82.x.x.x 1 track 1
sla monitor 1
type echo protocol ipIcmpEcho 81.x.x.x interface outside
num-packets 3
frequency 10
sla monitor schedule 1 life forever start-time now
router rip
network 192.168.32.0
redistribute static metric 1
version 2
no auto-summary
If the Internet on ASA 2 goes down, the default-route to ASA 1 will work.
Hope it helps.